1. Field of the Invention
This invention relates to an improvement of a dry-type integral multilayer analytical element for analysis of a particular component in a liquid sample. More particularly, this invention relates to an integral multilayer analytical element useful for analysis of an aqueous liquid sample such as for a clinical test where a body fluid is employed as a sample.
2. Description of prior arts
Recently, as a dry-type analytical element, many integral multilayer analytical elements wherein a water-absorptive reagent layer and a porous spreading layer are provided on a light-transmissive support have been proposed. The reagent layer contains reagents for color-forming reaction and a hydrophilic polymer binder, and the porous spreading layer is located at the topmost layer. In such multilayer analytical elements, an aqueous liquid sample spotted on the spreading layer spreads in this layer, and it reaches the reagent layer. Then, it reacts with the reagents to produce a detectable species such as a colored substance. The object substance such as a particular component of a body fluid can be determined by measuring this detectable species.
As the means to measure the detectable species it is known that a porous light-reflecting layer is provided above the reagent layer (U.S. Pat. No. 3,992,158). In the case of this analytical element, light is irradiated from under the support, and the reflected light is measured. Precision of this method is raised because influence of the suspensoids such as blood cells which inhibit the measurement can be removed. As the light-reflecting layer, various blushed polymer layers are disclosed in the specification of the above patent. They include a microporous blushed polymer layer formed by the deposition of polycarbonate particulates, polyamide particulates, cellulose enter particulates, etc., such as microporous filter membrane "Millipore" or "Metricell" formed by blushed polymer of cellulose enter, and a microporous layers where particulates of titanium dioxide, barium sulfate, etc. are dispersed in a binder such as cellulose acetate, polyvinyl alcohol or gelatin.
On the other hand, it is also known that a radiation-blocking layer is provided under the reagent layer (U.S. Pat. No. 4,042,335). In the case of this analytical element, the detectable species produced in the reagent layer passes through this radiation-blocking layer, accumulates in the detection layer located thereunder. Light irradiated from under the support is blocked by the radiation-blocking layer. That is, it is intended that the transmittance at the wave length to be measured is made less than about 1.0% (optical density is larger than about 2.0.), and influence of the reagent layer on the measurement of the detectable species can be nullified. The material of the radiation-blocking layer can include a matrix containing particulates of an inorganic pigment such as carbon black, titanium dioxide, zinc oxide or barium sulfate dispersed therein and the blushed polymer layer in which the above inorganic pigment is added, can be utilized.
However, when the light-reflecting layer is provided above the reagent layer, influence of the reagent layer largely appears. On the other hand, in the case that the radiation-blocking layer is introduced, the sensitivity of the measurement is lowered because the amount of the detectable species passing through this layer remarkably decreases. Particularly, when a light-reflecting pigment such as titanium dioxide particulates is employed for the light-reflecting layer or the radiation blocking layer, it is necessary to increase its content in order to secure the light reflectance or the radiation-blocking. As a result, it becomes difficult to form the above layers uniformly, and the physical strength of the layers becomes weak because the content of the polymer binder in the layers lowers.